This invention relates to a multi-element antenna transmit system for aircraft collision avoidance. More specifically, it relates to method and apparatus for calibrating differential cable delays in the antenna transmit system of a Traffic/alert Collision Avoidance System (TCAS).
The TCAS equipment located aboard a protected aircraft periodically transmits interrogation signals which are received by transponders located aboard other aircraft -- hereinafter referred to as target aircraft -- in the vicinity of the protected aircraft These transponders are preferably Mode S altitude reporting transponders so that TCAS capabilities can be fully exploited. In reply to the interrogation signals, the target aircraft's transponder transmits a response signal. The TCAS equipment aboard the protected aircraft determines the range of the target aircraft by using the round trip time between transmission of the interrogation signal and receipt of the response signal. In addition, if a Mode S transponder is used in the target aircraft, the protected aircraft can determine the altitude of the target aircraft from the response signal contents.
The TCAS typically uses a multi-element antenna array, for example, a four element array having each element arranged at 90.degree. spacing around the circumference of the array. Various components in the TCAS equipment, such as cables and receivers, are interfaced to each antenna element. The signals to be transmitted by an antenna element are delayed in phase as a result of passing through these components. The differential delays resulting from the components must be calibrated out to obtain accurate results.
A transmit system having autocalibration for use in a collision avoidance system, therefore, is needed.